Temperature compensated fiber optic pressure sensor

Abstract

A pressure sensor comprises a chamber formed from two members micromachined in silicon or a similar substance. The members define a chamber with at least one pressure sensitive membrane and an optic fiber extending through the chamber parallel to the membrane. The membrane may have an optical grating formed thereon which may be coated with a surface plasmon supporting substance. Light is injected into the fiber with a wavelength that couples with the grating on the membrane, either in a Bragg relationship or in coupling to a surface plasmon. The coupling, and thus the light lost from the fiber, varies with separation between the membrane and fiber and thus with the pressure outside the chamber. The members also include a thicker wall which is not pressure sensitive but which couples with an identifiably distinct portion of the light in the optic fiber to provide a temperature compensated reference. The pressure sensor can be made very small and rugged for combustion chamber pressure sensing in an engine and for other uses where very small pressure sensors are required.

The embodiments of the invention in which an exclusive property or privilege is claimed are defined as follows: 
     
       1. A pressure sensor comprising, in combination: wall means comprising a a micromachinable material subject to dimensional changes with temperature variation and enclosing a cavity with a constant reference pressure therein, the wall means further comprising a membrane portion effective to deflect in response to changes in pressure outside the cavity and changes in ambient temperature and a thick portion effective to deflect in response to changes in ambient temperature but not in response to changes in pressure outside the cavity;   optic fiber means extending through the cavity adjacent the membrane and thick portions of the wall means;   means effective to inject light into the optic fiber means for conduction therethrough;   light coupling means on the membrane and thick portions of the wall means within the cavity, the light coupling means on each of the membrane and thick portions of the wall means being effective to couple with identifiably distinct portions of the light being conducted through the adjacent optic fiber means so that some of each distinct portion of the conducted light escapes from the optic fiber means in amount variable with the separation between the fiber optic means and the coupled portion of the wall means;   light detection and signal generating means effective to sense the amounts of the identifiably  distinct portions of the light conducted through the optic fiber means and generate an output signal using the amount from the portion coupled with the thick portion of the wall means as a temperature compensated reference for the amount from the portion coupled with the thin wall means.   
     
     
       2. A pressure sensor according to claim 1 in which the identifiably distinct portions of light comprise light of two different wavelengths in a single optic fiber. 
     
     
       3. A pressure sensor according to claim 1 in which the optic fiber means comprises two separate optic fibers, the light in each of the separate optic fibers comprising the identifiably distinct portions of light. 
     
     
       4. A pressure sensor according to claim 1 in which the thick and membrane portions of the wall means are provided with optical grating means of different periodicities, each periodicity corresponding to a wavelength of light in the optic fiber means. 
     
     
       5. A pressure sensor according to claim 1 in which the thick and membrane portions of the wall are each provided with an optic fiber having a core in coupling relationship with one of the identifiably distinct portions of light in the optic fiber means.